Principally of tropical distribution, Ceratophyllum muricatum is known in North America north of Mexico only from ephemeral habitats in coastal southeastern United States. The wide range of variation of spines on the fruits in C . muricatum has led to the describing of several variants (typically when spine lengths have been reduced) as different species. As in C . demersum , spineless phenotypes from North America have been called C . submersum Linnaeus, which does not occur in the New World. The affinity of C . muricatum for shallow, ephemeral habitats results in its sporadic and nonpersistent occurrence in present North American localities. Fossil records of C . muricatum from the lower and middle Eocene document its occurrence in more inland sites, presumably when the climate of the interior stations was more similar to tropical conditions. Of the three North American species of Ceratophyllum , this species is most likely to be collected (in season) with fruit present.